There is an increasing interest in bone loss that occurs due to age, osteoporosis, or metabolic bone diseases. The impact of osteoporotic fractures on health care resources is significant at the present and is expected to increase even further. The necessity for being able to diagnose bone mineral content is accentuated by the therapeutic efforts made to restore or at least decelerate bone loss.
In the prior art, bone mineral content measurement techniques have included using radiographic methods, computerized tomographic methods, neutron activation analysis methods, single and dual photon absorptiometry methods and Compton absorptiometry methods. Recently, gamma cameras have been used for planar imaging in dual photon absorptiometry with a point source and a converging collimator.
For bone mineral mapping of the vertabrae and pelvic bones, which account for the largest number of fractures in adults, the most accurate methods are CT and dual photon absorptiometry. Use of single photon absorptiometry is practically limited to the forearm or humerus, femur or tibia, where the organ can be placed in a box filled with tissue equivalent material in order to limit the number of unknown parameters in the absorption equations to the bone mineral content only. Nevertheless, the accuracy of the single-photon measurement is still inferior and its clinical use is also questionable; since, there is little correlation between bone loss in the appendicular and the axial skeleton.
There is now available literature describing various bone mineral content mapping techniques and their limitations. See for example, "Nuclear Medicine: "Quantitative Procedures". by Wahner H W, Dunn W L, Thorsen H C, et al, published by Toronto Little, Brown & Co., 1983, (see pages 107-132). An article entitled "Assessment of Bone Mineral Part 1", appeared in the Journal of Nuclear Medicine, pp 1134-1141, (1984). Another article entitled "Bone Mineral Density of The Radius" appeared in Vol. 26, No. 11, (1985) Nov. Journal of Nuclear Medicine at pp 13-39. Abstracts on the use of gamma cameras for bone mineral content measurements are (a) S. Hoory et al, Radiology, Vol. 157(P), p. 87 (1985), and (b) C. R. Wilson et al, Radiology, Vol. 157(P), p. 88 (1985).
A prior art patent teaching a system for single photon absorptiometry is entitled "Bone Mineral Analyzer" and issued on Feb. 6, 1973 as U.S. Pat. No. 3,715,558. Another patent teaching a system for dual photon absorptiometry and indicative of the prior art is U.S. Pat. No. 3,996,471, entitled "Method and System for In-Vivo Measurement of Bone Tissue Using a Two Level Energy Source".
Many problems are encountered in the use of dual photon absorptiometry with a single detector. For example, it requires motion of the detector and the source in a very precise geometry; also, it is wasteful in terms of radiation dose to the patient, and time consuming in terms of locating the exact region of the bone.
The use of gamma cameras with point sources in planar dual photon absorptiometry solves the problems mentioned above. A remaining problem, however, is that the gamma rays impinge the body at a variety of angles. This means a two directional dispersion of the pathways of the gamma rays through the bones.
The dual photon absorptiometry measurements appearing in the literature primarily use the 153Gd isotope as a source of two gamma lines of 47 and 100 keV, with a fixed ratio of intensities of these two lines. Since the absorption of the lower line in the body is much stronger, the gamma detector sees a very large number of the high energy gamma rays compared to the low energy ones. In order to optimize the statistical accuracy of the measurement for a given total number of gamma rays of the given energies, a certain ratio of the intensities is required, not necessarily that dictated by the isotope.
The planar dual photon absorptiometry imaging with a single detector or with a gamma camera both have the handicap of measuring the collapsed projection of the body on the detector. This means that in cases where there are two layers of minerals over each other at the same position (e.g. The aorta calcification and the lumbar vertabrae), the measurements can not separate one from the other. Similarly, a planar image of the bone provides no information about its morphology, and therefore the planar images of cortical and trabecular bones are not separated. In particular, information as to whether bone loss occurs on the endosteal or periosteal bone envelope cannot be obtained.